Low rank coal, such as brown coal, is generally used only in limited areas near collieries because its high water content increases the transfer cost, which is disadvantageous from an economic standpoint, and further it is liable to ignite spontaneously during the transfer or storage thereof because of its high activity.
Under such circumstances, various proposals have been made to decrease the water content of such a low rank coal and to prevent it from spontaneous combustion.
As techniques to decrease the water content of coal, (1) a vaporization method and (2) a mechanical dehydration method, for example, are known. Also, as techniques to prevent the spontaneous combustion of coal, (1) an air shielding method (such as coal storage in water, coating of coal surface, covering of coal surface, compressive storage of coal, and inert gas sealing), (2) a cooling method, (3) a method of removing fine coal powder, (4) a briquetting method, and so forth are known. In more detail, a method in which coal is dried, heated in the presence of steam, and heat molded under atmospheric presure to produce briquette (see Japanese Patent Application Laid-Open No. 104996/1981) and a method in which coal is dried, heated rapidly, and then cooled rapidly (see Japanese Patent Application Laid-Open No. 149494/1981) are known.
These methods, however, are not satisfactory since no sufficient effect can be obtained and the operation is complicated.